1. Field of the Invention
The present invention relates to solid-state image sensing devices, methods for fabricating solid state image sensing devices and an image capture system using the same.
2. Description of Related Art
Integrated circuit image sensors are finding applications in a wide variety of fields, including machine vision, robotics, guidance and navigation, automotive applications, and consumer products such as digital camera and video recorders. Imaging circuits typically include a two dimensional array of photo sensors. Each photo sensor includes one picture element (pixel) of the image. Light energy emitted or reflected from an object impinges upon the array of photo sensors. The light energy is converted by the photo sensors to an electrical signal. Imaging circuitry scans the individual photo sensors to readout the electrical signals. The electrical signals of the image are processed by external circuitry for subsequent display.
Modern metal oxide semiconductor (MOS) design and processing techniques have been developed that provide for the capture of light as charge and the transporting of that charge within active pixel sensors and other structures so as to be accomplished with almost perfect efficiency and accuracy.
One class of solid-state image sensors includes an array of active pixel sensors (APS). An APS is a light sensing device with sensing circuitry inside each pixel. Each active pixel sensor includes a sensing element formed in a semiconductor substrate and capable of converting photons of light into electronic signals. As the photons of light strike the surface of a photoactive region of the solid-state image sensors, free charge carriers are generated and collected. Once collected the charge carriers, often referred to as charge packets or photoelectrons are transferred to output circuitry for processing.
An active pixel sensor also includes one or more active transistors within the pixel itself. The active transistors amplify and buffer the signals generated by the light sensing element to convert the photoelectron to an electronic signal prior to transferring the signal to a common conductor that conducts the signals to an output node.
Active pixel sensor devices are fabricated using processes that are consistent with complementary metal oxide semiconductor (CMOS) processes. Using standard CMOS processes allows many signal processing functions and operation controls to be integrated with an array of active pixel sensors on a single integrated circuit chip.
Refer now to FIGS. 1a-1c for a more detailed discussion of a pinned photodiode active pixel image sensor of the prior art. A substrate 5 heavily doped with a P-type impurity has its surface further doped with a complementary impurity to create a lightly doped P-type epitaxial layer 10. The photo detector regions 15a, 15b, and 15c are formed within the surface of the epitaxial layer 10 of the substrate 5. A P-type material is heavily diffused relatively deeply into the surface of the epitaxial layer 10 of the substrate 5 to form the P-well diffusions 25a and 25b. A P-type material is diffused into the surface of the substrate 5 to form the contact diffusions 50 for the P-well diffusions 25a. 
A gate insulator or thin oxide 95 is placed on the surface of the substrate 5 and polycrystalline silicon is formed on the surface to form the transfer gates 35a, 35b, and 35c and the reset gates 40a, 40b, and 40c. An N-type material is heavily diffused into the surface of the P-well diffusions 25a and 25b of the substrate 5 to form the floating diffusions 30a, 30b, and 30c and the N+ source/drain regions 45a, 45b, and 45c. The photo detector regions 15a, 15b, and 15c, the transfer gates 35a, 35b, and 35c, and the floating diffusions 30a, 30b, and 30c are transfer gate switches. The floating diffusions 30a, 30b, and 30c, reset gates 40a, 40b, and 40c and N+ source/drain regions 45a, 45b, and 45c form the reset gate switch.
The transfer gates 35a, 35b, and 35c of the transfer gate switches are connected to a transfer gating signals T_GT 65 and the reset gates 20a, 20b, and 20c of the reset gate switches are connected to the pixel reset signal PIX_RST 70. The N+ source/drain regions 45a, 45b, and 45c are connected to a power supply voltage source VDD. The floating diffusion 30a is connected to the gate of the CMOS transistor 80. The drain of the CMOS transistor 80 is connected to the power supply voltage source VDD and the emitter of the CMOS transistor 80 is connected to the drain of the CMOS transistor 75. The gate of the CMOS transistor 75 is connected to the row select signal 85. The CMOS transistor 75 acts as a source follower to buffer the electrical signal created by the photoelectron charge collected in the floating diffusion 30a. 
The photons that impinge upon the photo detector 15a are converted to photoelectrons and collected within the photo detector 15a. At the completion of an integration of the collection of the photoelectrons, the transfer gate 35a is activated to turn on the transfer gate switch to transfer the collected photoelectrons to the storage node of the floating diffusion 30a. When the collected photoelectrons are retained at the floating diffusion 30a the row select signal 85 is activated to turn on the transistor 75 to gate the pixel output electrical signal PIX_OUT 90 to external circuitry for processing and display. The amplitude of pixel output electrical signal PIX_OUT 90 is indicative of the intensity of the light energy hν or the number of photons 60 absorbed by the pinned photodiode. Once the pixel output electrical signal PIX_OUT 90 is read out the pixel reset signal 70 is activated to turn on the reset gate switch and the photo detector region 15a and the floating diffusion storage node 30a are emptied of the photoelectrons.
As is known in the art, a video display is formed of an array of picture elements or pixels. A pixel is one of the smallest complete elemental dots that make up the representation of a picture on a display. Usually the dots are so small and so numerous they appear to merge into a smooth image. The color and intensity of each dot is variable. In color displays the pixels are formed of red, green, and blue sub-pixels that are of a size and arrangement that light emitting from them is added to form the color of the whole pixel. Pixels are either rectangular or square.
U.S. Pat. No. 6,903,754 (Brown-Elliott) teaches an arrangement of color pixels for full color imaging devices with simplified addressing referred to as the Pentile Matrix. The architecture of the array consists of an array of rows and column line architecture for a display. The array consists of a plurality of row and column positions and a plurality of three-color pixel elements. A three-color pixel element can comprise a blue emitter, a pair of red emitters, and a pair of green emitters. The blue emitter is placed in the center of a square formed of the pairs of red and green emitters. The pair of red emitters are on opposing corners of the square and the pair of green emitters are adjacent to the red emitters and the other opposing corners of the square.
Image sensors (either CMOS or Charged Coupled Devices) often employ color filter arrays to generate the color components that are to be displayed. The color filter arrays, such as the Bayer Pattern as shown in U.S. Pat. No. 3,971,065 (Bayer), provide the color information or an image. However, this information must be reformatted to match the sub-pixel arrangement of a display.
“A CMOS Image Sensor with a Double-Junction Active Pixel”, Findlater, et al., IEEE Transactions on Electron Devices, January 2003, Vol.: 50, Issue: 1, pp.: 32-42, describes a CMOS image sensor that employs a vertically integrated double-junction photodiode structure. The imager allows color imaging with only two filters. The sensor uses a 6-transistor pixel array.
U.S. Pat. No. 5,028,970 (Masatoshi) provides an image sensor for sequentially reading signals from photoelectric converting elements disposed in a matrix and formed on a substrate in which both an image sensor and a photometry sensor are incorporated. The sensor includes a light-shielding layer disposed over the area of the substrate except the area of the photoelectric elements, the light-shielding layer forming a lower electrode. A PN-junction photodiode layer is disposed over the light-shielding layer, and an upper transparent electrode layer is disposed at least over the photodiode layer. The upper transparent electrode layer is divided into a plurality of pattern areas. If desired, at least one of the pattern areas of the upper transparent electrode layer may be further divided into a plurality of very small areas and color filters formed over the very small areas.
U.S. Pat. No. 6,111,300 (Cao, et al.) teaches a multiple color detection elevated pin photodiode active pixel sensor formed on a substrate. A diode is electrically connected to a first doped region of the substrate. The diode conducts charge when the diode receives photons having a first range of wavelengths. A second doped region conducts charge when receiving photons having a second range of wavelengths. The photons having the second range of wavelengths pass through the diode substantially undetected by the diode. A doped well within the substrate conducts charge when receiving photons having a third range of wavelengths. The photons having the third range of wavelengths pass through the diode substantially undetected by the diode.
U.S. Pat. No. 6,486,911 (Denyer, et al.) describes an optoelectronic sensor with shuffled readout. The optoelectronic sensor is a multi-spectral image array sensor that senses radiation of different wavelengths e.g. different colors. The array has at least one row of cells containing a plurality of series (R,G) of pixels which series are interspersed with each other. Each series consists essentially of pixels for sensing radiation of substantially the same wavelength e.g. the same color. At least two horizontal shift registers are provided each register being coupled to pixels of a respective one of the plurality of series (R,G) of pixels so as to enable the outputs from the pixels of each series to be read out consecutively at an array output. The pixels are preferably arranged in a Bayer matrix of Red, Green and Blue pixels and two interleaved shift registers are provided for reading out the pixel outputs for each color consecutively, in each row.
U.S. Pat. No. 6,693,670 (Stark) provides a multi-photodetector unit cell, which includes a plurality of light-detecting unit cells and a single charge-integration and readout circuitry. Typically, each of the cells produces charge representative of the detected light. The integration and readout circuit may be shared by the plurality of unit cells, and used to read-out the charge in real-time. The cluster may also include a switch associated with each unit cell, such that each switch connects its associated unit cell to the circuit. Each unit cell includes a photodetector, a photodiode or a photogate. The circuit includes a shared storage device, a shared reset circuit, or a readout circuit. Typically, the shared storage device may be for accumulating the charge in the focal plane.